1. Field of the Invention
The present invention relates to chemical mechanical planarization (CMP) techniques and, more particularly, to the efficient, cost effective, and improved CMP operations.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform chemical mechanical planarization (CMP) operations. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material grows. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then, metal CMP operations are performed to remove excess metallization.
A chemical mechanical planarization (CMP) system is typically utilized to polish a wafer as described above. A CMP system typically includes system components for handling and polishing the surface of a wafer. Such components can be, for example, an orbital polishing pad, or a linear belt polishing pad. The pad itself is typically made of a polyurethane material or polyurethane in conjunction with other materials such as, for example a stainless steel belt. In operation, the belt pad is put in motion and then a slurry material is applied and spread over the surface of the belt pad. Once the belt pad having slurry on it is moving at a desired rate, the wafer is lowered onto the surface of the belt pad. In this manner, wafer surface that is desired to be planarized is substantially smoothed, much like sandpaper may be used to sand wood. The wafer may then be cleaned in a wafer cleaning system.
FIG. 1A shows a linear polishing apparatus 10 which is typically utilized in a CMP system. The linear polishing apparatus 10 polishes away materials on a surface of a semiconductor wafer 16. The material being removed may be a substrate material of the wafer 16 or one or more layers formed on the wafer 16. Such a layer typically includes one or more of any type of material formed or present during a CMP process such as, for example, dielectric materials, silicon nitride, metals (e.g., aluminum or copper), metal alloys, semiconductor materials, etc. Typically, CMP may be utilized to polish the one or more of the layers on the wafer 16 to planarize a surface layer of the wafer 16.
The linear polishing apparatus 10 utilizes a polishing belt 12 in the prior art, which moves linearly in respect to the surface of the wafer 16. The belt 12 is a continuous belt which is cycled by rollers (or spindles) 20. The rollers are typically driven by a motor so that the rotational motion of the rollers 20 causes the polishing belt 12 to be driven in a motion 22, which is linear with respect to the wafer 16. The wafer 16 is held by a wafer carrier 18. The wafer 16 is typically held in position by mechanical retaining ring and/or by vacuum. The wafer carrier positions the wafer atop the polishing belt 12 so that the surface of the wafer 16 comes in contact with a polishing surface of the polishing belt 12.
FIG. 1B shows a side view of the linear polishing apparatus 10. As discussed above in reference to FIG. 1A, the wafer carrier 18 holds the wafer 16 in position over the polishing belt 12. The polishing belt 12 is a continuous belt typically made up of a polymer material such as, for example, the IC 1000 made by Rodel, Inc. layered upon a supporting layer. The support layer is generally made from a firm material such as stainless steel. The polishing belt 12 is rotated by the rollers 20 which drives the polishing belt in the linear motion 22 with respect to the wafer 16. In one example, an air bearing platen 24 supports a section of the polishing belt under the region where the wafer 16 is applied. The platen 24 can then be used to apply air against the under surface of the supporting layer. The applied air thus forms an controllable air bearing that assists in controlling the pressure at which the polishing belt 12 is applied against the surface of the wafer 16.
Unfortunately, in typical CMP systems, when a circular object such as a wafer, for example is pressed down upon a surface, which is rectangularly shaped such as the stretched polishing pad 12, uneven stretching of the pad surface may occur which is akin to a ripple effect. This is due to uneven nonlinear forces acting on the rectangular surface. A central portion is stretched and the edges of the rectangular surface are not stretched so the sides of the rectangular surface are up. The air bearing platen may be utilized to try to smooth the ripple effects and reduce the uneven stretching by applying higher air pressure to the polishing pad, but this results in significant increase in air consumption and still does not result complete elimination of the ripple effects, especially in the wafer edge area.
FIG. 1C illustrates the ripple effect in a static environment where a wafer 16 is pressed against a linear polishing pad 12. A loaded wafer, pressing over the elastic surface of the polishing pad causes a transient pad deformation zone near a wafer edge, which, being accompanied with the wafer relative tangential motion, creates a quickly attenuating longitudinal-transversal pad deformation wave. This results in re-distribution of pad-wafer contact force, affecting the removal rate and resulting in the edge effect. The forces causing the removal rate variations are shown by force arrows 26 and 28. Removal variations of up to 50% from the average may be observed due to the edge effects.
Linear belt CMP technology as described in FIGS. 1A and 1B has a reasonably flexible and stretchable polishing surface. The air bearing pad support utilized in the linear belt CMP provides a capability for manipulation of the pad shape and the contact force distribution enabling the minimizing of the edge effects up to 2 mm of edge exclusion. Unfortunately, one of the significant disadvantages of the air bearing is the circular symmetry of both upper surface and air providing orifices, which leads to high air consumption. During a CMP process, when the wafer 16 is pushed onto the polishing pad 12, the pad 12 deforms where a plurality of ripples 24 are formed. The ripples 24 are portions of the polishing pad 12 which moves up from its previous position due to the pressure applied by the wafer. The portions of the polishing pad 12 that are moved up exerts greater polishing force on the wafer 16. The effects of the ripples 24 at the edge of the wafer are especially pronounced resulting in an edge effect (removal variations at the wafer edge) where edge polishing rates are significantly higher than polishing rates at the center of the wafer 16.
FIG. 1D shows polishing effects of the ripples that may be formed when the non-rotating (static) wafer 14 is pressed down onto the polishing pad 12. Therefore, because of the aforementioned ripple effect, certain portions of the wafer as shown by areas 32 are polished more than the remaining areas of the wafer 16.
FIG. 1E shows polishing effects of the ripples when an air bearing platen is utilized underneath a polishing pad. In this example, an air bearing platen blowing air underneath a center portion of the polishing pad pushes up on the polishing pad where a center portion of the wafer is typically polished. The ripples are therefore reduced by the air pressure and wafer polishing in the wafer center is not as pronounced as shown in FIG. 1D. Therefore, less portions of the wafer 14 have uneven polishing. Unfortunately, usage of typical air bearing platen do not enable correction of excessive polishing in a plurality of areas 40 as shown in FIG. 1E.
As a result, because of the rectangular shape of a typical linear polishing belt and its interaction with a circular distortion from the air bearing creates a non-linear pad stretching field resulting in surface rippling which finally results in uneven polishing of the wafer due to uneven polishing pressure applied by different portions of the polishing pad.
Therefore, there is a need for a method and an apparatus that overcomes the problems of the prior art by having an apparatus that may be utilized to correct stress distribution in a polishing pad so polishing pressure applied by the polishing pad to the wafer is consistent through different sections of the wafer. Such an apparatus additionally stretch the under-stretched belt sections to enable more consistent and effective polishing in a CMP process without requiring large air consumption.